1. Field of the Invention
This invention relates to self-priming centrifugal pumps and safety installations therefor.
2. Brief Description of the Prior Art
In nuclear pumping installations, for example, all of the apparatus which process highly radioactive and very toxic liquids and those which are designed to reprocess irradiated fuels, are surrounded with protective screens. Such screens essentially serve to stop the radiation and prevent the dissemination of radioactive and toxic gases and aerosols into the working zone where personnel are present.
Furthermore, in order to facilitate maintenance operations of this enclosed cell apparatus, precautions are taken so as to limit, as much as possible, contamination of the internal walls of the protective cell and the external walls of the apparatus contained thereby. The apparatus and, in particular, the tanks are, for example, entirely closed and their vent-holes are connected by air-tight pipes to installations for the monitoring and treatment of the gaseous effluents.
In order to eliminate leak hazards in the lower parts of the installation, for example, welded assemblies are compulsory and assemblies provided with flanges are forbidden. Accordingly, the installation of centrifugal pumps at the bottom of the tanks is, therefore, not possible, and it is for this reason that self-priming centrifugal pumps have been operating for the past few years.
The installation of such pumps is above the draw tanks and this conforms more satisfactorily to existing safety standards and, moreover, facilitates maintenance operations.
French Pat. No. 1,344,957 issued to applicant describes a self-priming centrifugal pump. The pump is described as having a vertical axis and comprises a body separated on the inside by a partition into two superimposed communicating chambers. The lower chamber is connected to a suction pipe and the upper chamber to a delivery pipe. This pump has a centrifugal wheel located between the two chambers which is driven about the vertical axis and is controlled by a motor located above the enclosure. Such wheel comprises a horizontal plate provided on each of its faces with a series of blades or fins, the respective dimensions of which are determined so that the dynamic pressure created by the fins of the internal face of the plate will be lower than that created by the fins of the upper face. A pipe issuing at a short distance from the bottom of the lower chamber puts it into communication with the central part of the region located below the lower face of the wheel plate, and a pipe surrounds the driving axis and pulls it into communication the central part of the region located above the upper face of the plate with the outside of the body of the pump.
The rotation of the centrifugal wheel ensures, under these conditions, the transfer of the liquid from the lower chamber or suction chamber to the upper chamber or delivery chamber by means of the fins provided on the lower face of the plate while preserving, by means of the fins of the upper face, the tightness between the external atmosphere and the delivery chamber without the use of static joints and/or a stuffing-box. Indeed, the fluid seal in this instance consists of the liquid itself, under the action of the centrifugal force due to the fins provided on the upper part of the plate forming a stable ring which is maintained between these fins and the pipe surrounding the drive shaft.
Fench addition Pat. No. 88,688 to the above-noted French patent relates to various improvements introduced into a self-priming pump of the previously described type. In particular, the pump includes blades providing a vacuum pump operation during the priming phase, one or more communication ports between the suction chamber and the delivery chamber in order to preserve, at shut-off time, a sufficient quantity of liquid for the following start-up, and a device for effecting the connection to the pipes which also provides for disassembly at a distance.
Furthermore, German Pat. No. 1,190,794 discloses a centrifugal pump with a vertical axis which can be easily disassembled due to the fact that it comprises a fixed bowl shaped body, closed on its lower part and enlarged in the form of a frustum on its upper part. It is described that the enlarged part acts as a seat for a removable block which comprises the pump wheel and its cage, as well as a motor for driving the pump.
However, the use of these pumps still has several drawbacks. For instance, tightness is only completely attained during operation. Consequently, under non-operating conditions, a certain quantity of air containing radioactive aerosols can leave the pump during the start-up or shut-off periods.
Moreover, in installations for the reprocessing of irradiated fuels and, in particular, for the so-called PUREX process, which is the most often utilized, organic solvents, such as tributyl phosphate, are used. The surface tension of such types of solvents, however, in the presence of air is much greater than that of water. Accordingly, this relationship prevents the self-priming pumps of the known type from pumping the above-described solvents because a very stable foam is formed at start-up which prevents operation. What is, however, more serious is the fact that even a small quantity of solvent even whenever accidentally mixed with an aqueous solution, which normally does not contain it, is enough to prevent start-up.